MOS transistors have achieved higher integration and increase in current driving force by miniaturization in device dimensions, particularly, reduction in gate length. However, when the gate length is simply reduced, a problem of the short channel effect occurs. It is a phenomenon that the influence of a drain field is largely exerted on a channel just below a gate insulating film, so that off-state current increases, a threshold value sharply decreases, and variations increase. A structure of elevating a source/drain region on a silicon substrate in order to solve the problem is disclosed in a non-patent document 1.
On the other hand, it is pointed out that miniaturization of devices following the conventional trend will face to physical and economical obstacles in near future. It is therefore necessary to establish a performance improving technique by a method other than miniaturization.
A material having high carrier mobility may be used for the channel layer to improve the operation speed of a MOS transistor. It is known that when stress is applied to silicon crystal, due to a change in the band structure, scattering and effective mass decreases, and the mobility improves.
A non-patent document 2 proposes a technique of increasing the current driving force by receiving compressive strain in the channel direction by a structure in which silicon germanium with a lattice constant different from that of a silicon substrate is buried in a source/drain region, and raised from the silicon substrate. A non-patent document 3 describes that, in a MOS transistor of such a structure, the channel strain amount improves depending on the height of the elevation from the surface of the silicon substrate.
As another method, a method of forming a silicon nitride film as a stressed film on a MOS transistor is disclosed in a non-patent document 4. A non-patent document 5 describes that a cumulative effect is obtained by combination of the above techniques.
Non-patent document 1: IEDM Technical Digest, 1987, pp 590 to 593
Non-patent document 2: IEDM Technical Digest, 2003, pp 978 to 980
Non-patent document 3: IEDM Technical Digest, 2004, pp 1055 to 1058
Non-patent document 4: IEDM Technical Digest, 2004, pp 213 to 216
Non-patent document 5: SSDM Technical Digest, 2005, pp 32 to 33